Only way I could think of now is as below:
- Open 'customers list' window in Express Invoice.
- Select all or desired customers from the list.
- Click top 'Customer' menu and select 'Export to CSV', select a desired folder location and click OK.
- Now, open the generated CSV file with MS-Excel (or import in Outlook) or any other program of your choice.
(I was able to import all the desired email address and other fields in my Outlook contact by manually matching the field headings when importing from the CSV file.)
Hope this helps!